1. Field of the Invention
The present invention relates to the field of refrigerant, compressed gas/air dryer systems, and more particularly to a refrigerant receiving apparatus which meters and stabilizes the return of refrigerant in compressed gas/air refrigerant dryer systems.
2. Background and Prior Art
Presently, many industrial applications using air or gas driven machinery have a need for dry air or gas in the process of operating, product process, product fabrication, as well as many other applications. Air or gas driven machinery is most commonly operated using pressurized, i.e., compressed air or gas that contains water that can react on or condense within product or apparatus and negatively impact the air or gas usefulness. Moisture negatively impacts the product process systems by causing costly equipment maintenance or equipment failure and befouled product.
Typically, because of the compression process, compressed air or gas is saturated with 100% relative humidity; water in excess of the humidity capacity condenses as moisture on machine or product surfaces. When the volume is reduced, i.e., the air is pressurized or compressed, the dew point for water is approached or exceeded thereby causing condensation. Filters and traps only take out water (if any) already condensed out of the compressed air or gas as it has cooled. These filter/trap systems do nothing to remove the 100% relative humidity representing water in vapor phase still remaining in the compressed air/gas. As the air or gas cools condensation is exacerbated.
Refrigerant dryers are the most common means for acceptably removing moisture from compressed air or gas for industrial uses. The quality of the gas/air being dried at the dryers output is measured in terms of dew point; the lower the dew point temperature, the greater the dryness of the gas/air and thus the higher the quality of the process. Generally the acceptable dew point ranges is between 33° F. and 50° F. depending on the application. More water vapor in the feed air or gas and higher pressures from compression each make achieving acceptable dew point more difficult. There are several methods of refrigerant dryer operations; the following are some of the more conventional systems: Cycling Dryers, Non-Cycling Dryers, Mass Storage and Variable Speed Drive Dryers.
Simply described, refrigerant air or gas dryers use: i) a refrigerant compressor (with appropriate accumulator and receiver apparatus), ii) a series of heat exchanger vessels and/or other heat transfer apparatus, a condensing apparatus, and, iii) refrigerant process control apparatus (expansion, pressure regulating, hot gas valves, bypass valves; solenoids and electronic sensors/controls with or without variable speed drive (VSD) devices for the compressor motor; etc.). These systems each operate at various levels of efficiency, both with respect to cost and dew point performance.
Various refrigerant dryer devices have been designed to take the water vapor from the air, for example, the system of U.S. Pat. No. 5,207,072 to Arno features a cycling configured refrigerant air or gas dryer with an unloading means that allows a compressor motor to coast or free wheel during periods of low demand for refrigerant cooling. Thus, this advanced cycling dryer is said to be an energy savings dryer as compared to conventional non-cycling systems. Another example is a system configured with a VSD device to modulate the compressor during the period of lower demand for refrigerant cooling. A system so configured would be considered an energy saving dryer because the compressor is consuming less energy during the lull intervals.
However, each of these above listed devices use an expansion valve to feed or deliver compressed refrigerant into a heat exchange type vessel, where the expansion of refrigerant produces a coldest point for heat exchange purposes. Expansion valves rely on a temperature and pressure feedback which causes the valve opening to either increase in size for greater refrigerant feed, or, decrease in size for less refrigerant feed. These valves are conventionally available as strictly mechanical valves or in combinations of mechanical and electrical and/or electronic (solenoid, proportional, step motor drive, etc. with or without microprocessor control) valves; all having the desired goal to feed expanded refrigerant as required by the recycling means back to the refrigerant side of a heat exchanger providing a coldest point for thermal exchange.
One of the problems in this type of device is that the expansion valves are called-out in terms of tonnage (the capacity with which the device can deliver expanded refrigerant and feed the system). The tonnage is expressed as a range based on differential pressure; for example, 10 tons (generally for operating in systems from about 80,000 btus to about 120,000 btus capacity requirements). When these devices are specified in the design of a system, the tonnage expressed could actually be implemented as on the low side, in the mid range, or, on the high side of the valve capability to deliver refrigerant feed. This means, in simple terms, that the valve in any particular system may be required to work near maximum capacity, in a mid range, or barely working efficiently at low capacity, each, respectively in each design. That equates, in each of the scenarios, to the valve working less than ideal for most of the range of the system designs capacities.
Another problem that exists in conventional systems is flooding. Flooding refers to the ability to keep the evaporator refrigerant side full with liquid. As load on the system operates, the refrigerant boils off and returns; leaving the evaporator only partially flooded. In such a scenario, an evaporator could be partially filled with liquid and the remaining space filled with vapor or foam from the boiling instigated by the evaporator valve. Vapor or foam within the evaporator does not transfer heat as efficiently as the liquid does, thus the efficiency of the evaporator suffers because the contact surface area with liquid for heat exchange is less than ideal.
To broaden the range of efficient operation of the valves use in varied systems, the expansion valve is conventionally adjusted. The adjustment is expressed in terms of superheat; a value derivative equivalent to the refrigerant systems compressor suction pressure converted to degrees in temperature (as related to a specific refrigerant type) and subtracted from the refrigerant systems suction temperature.
An expansion valve may generally be used over a wide tonnage range. Thus factory adjustment for superheat is undesired. Each valve must be set for superheat to reflect 10-15° F. over room temperature. To set the superheat, one must use a thermocouple or thermometer to measure the temperature of the suction line, for example, at a thermal bulb. Then one measures the pressure in the suction line at the thermal bulb well or external equalizer. The measured suction is then converted to a pressure equivalent saturated temperature using a pressure temperature chart. The difference between this value and the temperature measured at the thermal bulb well is expressed as the superheat. Superheat is often in the approximate range of five to ten degrees F.
Ideally, the superheat (a value derived from suction temperature and suction pressure), would give feedback to the expansion valve to close-down (a call for less refrigerant) to maintain a predetermined level of performance. Conversely, when the call is to increase refrigeration, the change in superheat would cause the expansion valve to open-up and feed more expanded refrigerant.
Unfortunately, no valve devices work at an optimum under most or all conditions in any given system or design. In practice the parameters routinely overshoot. The result is an expansion valve hunting endlessly. That is the valve will open-up for more feed which will be followed by a close-down because of too much feed, and again, an open-up because of too little feed resulting in a drop of the flooding level; resulting in a never ending cycle. This phenomenon occurs in every system at some point even in carefully designed systems using the mid range as ideal or with sophisticated electronically assisted expansion valve devices. This hunting, over/under, constant pursuit to satisfy the endless loop of superheat feedback results in less than ideal performance of the gas/air dryer system desired to produce a low, constant dew point temperature gas or air. The hunting results from the refrigerant being returned in an erratic manner.
In U.S. Pat. No. 5,099,655 to Arno, the inventor directly addressed the negative result of having only a partially flooded evaporator. U.S. Pat. No. 5,099,655 teaches that having a suction line heat exchanger, as a flooding level control, effectively breaks-up liquid slugs of refrigerant return. This was accomplished by using the discharged refrigerant out of the compressor on one side of the flooding level control suction line heat exchanger, while the return flow is through the other side. The hot discharge tends to flash the liquid slug to its vapor state and thusly, reduces its effect on the expansion valve regulating bulb. The result is a higher level of liquid refrigerant in the evaporator. However, even this improved apparatus can still suffer from the overfeed/underfeed problem scenario.
U.S. Pat. No. 5,207,072 to Arno discloses an unloading structure for compressors of refrigerant systems, and, U.S. Pat. No. 5,099,655 teaches level control for refrigerant systems that use flooded shell evaporators.
U.S. Pat. No. 6,516,626 to Escobar, discloses a two stage refrigeration system incorporating a means for storing refrigerant vapor and slurry having a receiving tank or tanks. U.S. Pat. No. 6,490,877 to Bash, teaches parallel evaporators and a means to control the mass flow rate of the refrigerant to each evaporator. Reissue patent RE 33,775 to Behr, shows multiple evaporators and method of controlling the valve in a refrigeration system. However, the various prior systems are undesirable in that they do not provide precise control.
Thus the state of the art is clearly not ideal. Normal load changes during industrial cycles can adversely affect dryer operations, resulting in poor dew point performance, waste of energy and wear-and-tear of equipment. The industry has accepted that it is the nature of refrigerated gas/air dryer systems even those having sophisticated electronically assisted expansion valves to function with cyclical operation expansion and thus routinely experience the same over/under performance.
Thus it is readily apparent that there is a longfelt need for a means for metered or controlled refrigerant return from an evaporator system and for a process to maintain stable, balanced parameters affording a very high performance true flat-line in dew point of a gas/air dryer system, that is a means and process that sets forth a method for controlling the rate at which the metered return occurs and a system that will modestly modulate return (always track the demand and will output perfect dew point temperature according to the gas/air dryers load).